Glatiramoids are a family of synthetic copolymers containing four amino acids: L-glutamic acid, L-alanine, L-lysine and L-tyrosine, in a defined molar ratio (Varkony et al. 2009). Glatiramer acetate (GA) is a member of the glatiramoid family, and it is the active pharmaceutical ingredient of the commercially available medicine Copaxone® (Teva Pharmaceutical Industries Ltd., Israel), indicated for the treatment of patients with relapsing forms of multiple sclerosis.
According to the product labelling, GA consists of the acetate salts of synthetic polypeptides, containing four naturally occurring amino acids: L-glutamic acid, L-alanine, L-tyrosine, and L-lysine with an average molar fraction of 0.141, 0.427, 0.095, and 0.338, respectively. The average molecular weight of GA ranges from 5,000 to 9,000 daltons.
Chemically, GA is designated L-glutamic acid polymer with L-alanine, L-lysine and L-tyrosine, acetate (salt). Its structural formula is:(Glu,Ala,Lys,Tyr)x.xCH3COOH(C5H9NO4.C3H7NO2.C6H14N2O2.C9H11NO3)x.xC2H4O2                 CAS—147245-92-9        (NDA 020622/S-089 FDA Approved Labelling Text dated Jan. 28, 2014)        
GA is synthesised via amino acid polymerisation followed by a subsequent cleavage or partial depolymerisation step. Because of the stochastic nature of polymerisation and cleavage reactions, the polypeptides obtained by this process vary in sequence, length and molecular weight (MW), resulting in a characteristic molecular weight distribution (MWD) that spans from about 2,500 to 20,000 daltons.
Various analytical methods useful for characterising glatiramer acetate or similar polypeptide mixtures are described in patent literature and some regulatory documents (including “citizen petitions”). These methods include amino acid analysis, size exclusion chromatography, circular dichroism, reversed-phase liquid chromatography, capillary electrophoresis, peptide mapping, Edman sequencing, analysis of C- and N-terminal signatures (e.g. C-terminal diethylamide and N-terminal pyroglutamate).
The present invention is based on a new and original chromatographic method, which can distinguish glatiramer acetate from non-conforming copolymers and may be used, together with other quality attributes, to choose batches of glatiramer acetate suitable for pharmaceutical use.